1. Field of the Invention
The present invention relates to a lamination type ceramic heater, which is applied to an oxygen sensor and the like.
2. Related Arts
An oxygen sensor element generally holds a heater for heating it to be an activation temperature. As the heater, recently, a plate-like lamination type heater has been widely used. The lamination type heater is typically composed of a ceramic heater substrate having heat and lead portions thereon and a ceramic cover plate disposed on the heater substrate to cover the heat and lead portions.
This type of ceramic heater is disclosed, for example in WO 94/14057 and JP-A-7-35723. In WO 94/14057, the ceramic heater has two heater layers electrically connected to one another in series. In JP-A-7-35723, a percentage of a short side with respect to a long side of the ceramic heater in cross section is fixed to be 75%-100%.
This ceramic heater is manufactured in the following way. First, two ceramic green sheets for the substrate and for the cover plate are prepared. After several heater patterns for the heat portion and the lead portion are printed on the ceramic green sheet for the substrate, the ceramic green sheet for the cover plate is laminated with the ceramic green sheet for the substrate, thereby forming a lamination body.
Next, the lamination body is baked. The baked lamination body is cut to be divided into several baked intermediate bodies, each of which has a corresponding one of the heater patterns thereon. Thereafter, conductive paste is printed on side surfaces of each of the intermediate bodies. The printed portions of the conductive paste become to serve as side electrodes by baking. Finally, lead wires are brazed to the side electrodes, so that the ceramic heater is finished.
However, in the above-mentioned method, cutting of the baked lamination body causes a plurality of micro-cracks in the lamination body, i.e., in the intermediate bodies. When the ceramic heater works, the ceramic heater has temperature distribution. For example, a surface temperature at a portion corresponding to the integrated heater portion is approximately 1000.degree. C., and a surface temperature around the lead portions brazed to the side electrodes is approximately 400.degree. C. This temperature distribution generates thermal stress in the ceramic heater, which can grow the above-mentioned micro-cracks so that the ceramic heater is damaged.